1. Field of the Invention
This invention relates to telecommunications systems, and is concerned in particular with improving the quality of speech signals transmitted over telecommunications networks.
2. Related Art
Signals carried over telecommunications networks are subject to degradation from interference, attenuation, data compression, packet loss, and limitations in digitisation processes. It is desirable to monitor signals at intermediate points in their transmission paths to identify any imperfections and, if possible, to "repair", the signal; that is, to restore the original signal. Provided that this is done before the signal has degraded to the point where it is no longer possible to discern its content at all, the "repaired" signal can then be retransmitted. The process can be repeated as often as necessary, according to the length of the transmission path and the degree of degradation.
Data signals are comparatively easy to repair as they comprise a limited number of characters: (e.g. binary 1s and 0s; the twelve-character DTMF (dual tone multiple frequency) system, or the various QAM (quadrature amplitude modulation) constellations. Repair of such signals is carried out by identifying which of the "permitted" characters is closest to the degraded one actually received, and transmitting that character. For example, in a binary system, any signal value exceeding a threshold value may be interpreted as a "1", and any below the threshold as a "0". Check digits and other means may be included in the transmission to further improve the integrity of the transmission.
However, speech signals do not have a limited character set of this kind, and it is thus more difficult to identify automatically whether the signal has been degraded at all, still less how to restore the original signal.